Sixth nerve palsy

Definition

Cranial nerve six supplies the lateral rectus muscle allowing for outward (abduction) eye movement. A sixth nerve palsy, also known as abducens nerve palsy, is a neurological defect resulting from an impaired sixth nerve or the nucleus that controls it. This may result in horizontal double vision (diplopia) with in turning of the eye and decreased lateral movement.

Description

Isolated sixth nerve palsies usually manifest as a horizontal diplopia worse when looking towards the affected eye, with a decreased ability to abduct. Since the sixth nerve only innervates the lateral rectus muscle, isolated palsies will only manifest in this fashion.

Demographics

Sixth nerve palsies have no predilection for males or females and can occur at any age.

Causes and symptoms

For all intensive purposes causes of abducens nerve palsy can be classified as congenital or acquired. Isolated congenital sixth nerve palsy is quite uncommon. If congenital the usual presentation is accompanied by other cranial nerve deficits as seen with Duane's retraction or Moebius syndromes . Strabismus, commonly known as "lazy eye," may mimic the appearance of abducens nerve palsy and may go undetected until adulthood because of compensatory mechanisms allowing for alignment of the eyes when focusing. Abduction deficits may also result from myasthenia gravis , thyroid eye disease, inflammation and orbital fractures which imitate sixth nerve palsies.

A myriad of causes resulting in abducens nerve palsies have been reported. In order to better differentiate these one must take into account the patient's age and underlying illnesses. In children trauma and tumors were reported as the most common causes. Therefore if no trauma has occurred one must consider a tumor of the central nervous system in the pediatric population. Other causes include idiopathic intracranial hypertension, inflammation following viral illness or immunization, multiple sclerosis , fulminant ear infections, Arnold-Chiari malformations and meningitis.

New onset palsies in adults can stem from myasthenia gravis, diabetes, meningitis, microvascular disease (atherosclerotic vascular disease) or giant cell arteritis (arterial inflammation). Other causes include Lyme disease , syphilis, cancers, autoimmune disorders, central nervous system tumors, and vitamin deficiencies.

Children may be found to have head tilt or in-turning of the affected eye, with reduction of outward gaze. They will very rarely complain of double vision, while adults may describe two images, side by side (horizontal diplopia), which are furthest apart when looking towards the affected eye. Covering of one eye, no matter which one is covered, and gazing away from the affected eye will resolve their diplopia. Patients may also note muscle weakness, possibly heralding myasthenia gravis, or headache and jaw pain , raising the possibility of giant cell arteritis.

Optic nerve swelling or jumpy eye movements (nystagmus) may occur at any age and warrants immediate work-up for a central nervous system tumor.

Diagnosis

Diagnosis of sixth nerve palsy is based on history and clinical findings. Once the diagnosis has been established the work-up should be tailored based on the patient's age and medical history.

Pediatric patients with no apparent trauma should undergo magnetic resonance imaging of the brain with contrast enhancement to rule out a central nervous system structural lesion (tumor or aneurysm). If the imaging is without abnormal findings a lumbar puncture (spinal tap) should be done to exclude increased intracranial pressure or infection. If this is normal, consideration of a post-viral or post-immunization palsy may be safely entertained.

Isolated abducens palsies in the adult population should be approached in a more conservative manner. If a patient is known to have diabetes, high blood pressure, or atherosclerotic vascular disease, a small stroke is likely. If diplopia worsens or no improvement occurs at eight weeks time, a more extensive work-up including magnetic resonance imaging of the brain with contrast and blood work to exclude infections, autoimmune disorders, vitamin deficiencies, or inflammation is warranted. A potentially devastating, blinding disorder known as cranial arteritis may occur in patients usually over 50 years of age. Headache, jaw pain worsened with chewing, night sweats, fevers, weight loss, or muscle aches necessitate blood work to rule out this inflammatory disorder.

Treatment team

Ophthalmologists, neuro-ophthalmologists, optometrists, neurologists, and pediatricians are medical specialists who can evaluate and diagnose a patient with a sixth nerve palsy. Usually an optometrist or ophthalmologist will initially see a patient complaining of diplopia or displaying findings of sixth nerve palsy. A referral will then likely be made to a neurologist or neuro-ophthalmologist for evaluation and work-up.

Treatment

Treatment of sixth nerve palsies is dictated by the underlying causes. Older patients who are thought to have had a mini-stroke are observed for several months, because of likely spontaneous resolution. Causes related to masses of the central nervous system or systemic disease should be managed and treated promptly by the appropriate specialist.

Children who are at risk for amblyopia can be treated with patching to reduce the risk of permanent visual loss. Older patients may elect to use a prism incorporated into a spectacle to reduce or eliminate their double vision. Prisms or fogging of one eye are excellent options for the older patient being observed for spontaneous resolution of their palsy.

If diplopia persists for greater than six months and prisms cannot realign the images surgical intervention is an option. Depending on the amount of lateral rectus muscle function one or two surgical options are used. If muscle function remains, weakening of the medial rectus muscle and tightening of the affected lateral rectus muscle may resolve the patient's complaint. If no function exists then a muscle transposition surgery can help restore some abduction ability.

Botulinum toxin may also be used to weaken the medial rectus muscle of the affected eye. This weakening effect is short-lived and repeat injections are necessary.

Clinical trials

As of November, 2003, no clinical trials regarding abducens nerve palsies were underway.

Prognosis

Isolated abducens nerve palsies in the older population are usually related to a small stroke and resolve within several months. Palsies related to trauma or brain masses have a guarded prognosis and recovery, if any, may take up to one year. Treatment of systemic disorders, such as myasthenia gravis, have an excellent prognosis, while inflammation related to multiple sclerosis is likely to improve as well. Unfortunately there are no hard and fast rules regarding recovery of any sixth nerve palsy.

Special concerns

Patients afflicted with a sixth nerve palsy should refrain from driving unless an eye patch is used. In addition certain types of employment may warrant a medical leave or temporary change of duties.

Resources

BOOKS

Beers, Mark H., and Robert Berkow, eds. The Merck Manual of Diagnosis and Therapy. Whitehouse Station, NJ: Merck Research Laboratories, 1999.

Burde, Ronald M., Peter J. Savino, and Jonathan D. Trobe. Clinical Decisions in Neuro-Ophthalmology, 3rd ed. St. Louis: Mosby, 2002.

Liu, Grant T., Nicholas J. Volpe, and Steven L. Galetta. Neuro-Ophthalmology Diagnosis and Management, 1st ed. Philadelphia: W. B. Saunders Company, 2001.

Adam J. Cohen, MD